Genetic studies on the immune response continue to employ antibodies to streptococci which are produced in inbred rabbits. A unique feature of these antibodies is their remarkable molecular uniformity. Because of this uniformity, these antibodies are especially useful in examining the relationship among the genes which control antibody synthesis, and in exploring the nature of the mechanisms responsible for antibody diversity. For example, the detection of a similar molecular marker on the streptococcal antibodies from inbred rabbits and its absence among unrelated rabbits emphasizes the genetic control over antibody synthesis. A preferential association between the genes for this molecular marker and other genes points to multiple gene interactions in the synthesis of a specific antibody. Studies have continued on: 1) the genetic control of the immune response to streptococci in rabbits; 2) structural determinants of genetic markers; and 3) the specificity of the lgG and lgM anti-lgGs which occur in abundance in the antisera of rabbits immunized with streptococci.